1. Field of the Invention
The invention relates to electrostatic dissipative, semi-conducting polymeric composites. More specifically, the invention concerns static-dissipative polymeric composites consisting of an insulative polymeric based resin and an electrically active filler to make the resultant composition semi-conducting. The invention also relates to physical vapor deposition of a semi-conducting inorganic material onto the surface of particles.
2. Description of the Related Art
Traditional electrically active plastic composites use highly conductive fillers, such as particles, fibers or flakes in an insulative polymeric base resin. Commonly employed electrically active fillers include carbon powder, carbon fiber, metal powders, fibers and flakes, and metallized particles, fibers and flakes. These are disclosed in multiple patents, e.g., U.S. Pat. Nos. 4,634,865, and 4,288,352.
The filler must be used in an adequate volume for the individual particles or fibers to be in electrical contact in order for the resulting composite to be electrically conductive. The level of conductivity depends on the number of conductive paths created by the particles or fibers. Low levels of filler are ineffective, because there are few conductive paths. Therefore, to increase conductivity, the amount of filler must be increased. These techniques work well for composites in the conductive range (volume resistivity of 10.sup.1 -10.sup.4 ohm-cm) as this range falls in a region for highly conductive fillers where a small change in filler content has little effect on the conductivity of the composite because so many conductive paths exist.
However, using these techniques in the semi-conducting range (volume resistivity of 10.sup.4 -10.sup.14 ohm-cm) causes problems. For fillers of high conductivity, this range falls into a region where a small change in filler causes a large change in the conductivity of the composite. This makes the conductivity very difficult to control. This sensitive balance between the conductive filler and the insulative resin is further complicated by processing variations such as polymer/fiber orientation, density, shear rates and cooling rates.
Composites using highly conductive fillers also suffer from other detrimental characteristics:
In composites utilizing conductive fillers with a relatively high aspect ratio, i.e., fibers or flakes, the content of conductive filler to insulating polymer must be relatively low to control the number of connections. This results in greatly reducing the probability of providing a "ground" or an electrical path for a static charge to dissipate through.
In composites utilizing conductive powder fillers, e.g., carbon powder, as disclosed in U.S. Pat. Nos. 3,563,916 and 3,836,482, the composite exhibits "sloughing" where the powder filler rubs-off, out of the polymeric matrix.
In composites utilizing metals as the conductive filler, i.e., metal powders, fibers and flakes, as disclosed in U.S. Pat. No. 3,576,378, the metal particles are very dense compared to the polymer matrix and thus tend to separate from the matrix during processing resulting in a non-homogenous composite.
In composites utilizing conventional metallized particles, i.e., microspheres, microbubbles, fibers and flakes, the material coating is typically limited to solution processing techniques or "plating" where the coatings are relatively thick. Solution processing techniques limit the materials to those with high conductivities thereby yielding composites conductive rather than semi-conductive. In addition, plating technology has metal adhesion problems where the metal plating nodules pull away and separate from the host particle.
To be useful for semi-conductive composites, a filler must be less conductive than the "highly conductive" fillers, but not so resistive that an excessive volume loading is required to obtain the required conductivity, as this would have a negative effect on the mechanical properties of the composite.
Inorganic thin-film coating of metals onto particulate matter is known for a variety of purposes. U.S. Pat. Nos. 4,618,525, (Chamberlain et al.) and 4,612,242, (Vesley et al.) discloses glass microbubbles coated with a variety of metal oxides. Use of such bubbles as pigmenting fillers in pressure-sensitive adhesive tapes is disclosed. Electrical properties are not disclosed, nor is the use of such microbubbles as fillers in a plastic polymer.
Commonly used techniques to achieve controlled semi-conducting plastic composites consist of the incorporation of a semi-conducting material such as metal oxides (e.g., copper oxide) in particulate form into plastics. The particles are difficult to disperse and much denser than plastics resulting in heavy and expensive semi-conducting plastic composites.
Other controlled conductivity coated particles for anti-static plastics have been described in U.S. Pat. Nos. 4,373,013 and 4,431,764. These are titanium dioxide particles coated with antimony tin oxide. Again, these particles are difficult to disperse and much denser than plastics resulting in heavy and relatively expensive semi-conductive plastic composites. Further, the particles disclosed to be useful have an average particle size of 0.07.mu. to 0.14.mu..
U.S. Pat. No. 4,271,045, discloses an electrically conductive layer comprising a mixture of minute particles of semi-conductive material obtained through pyrolysis of a carbon-containing compound coated or doped with one or more Group III-VIII elements.
U.S. Pat. No. 4,175,152 discloses polymeric materials containing semiconducting pyropolymeric inorganic refractory oxide material having resistivity of from about 0.001 to about 10.sup.10 ohm-cm.
It has now been discovered that applying a thin-film coating of cupric oxide (CuO) on to the surface of microbubbles creates a semi-conducting filler that is easily dispersed in polystyrene at a 50 volume percent loading. The resulting composite exhibits superior electrical properties in the semi-conducting range (volume resistivity of 10.sup.4 -10.sup.14 ohm-cm), provides excellent "groundability" to dissipate electrical charge and provides very low density.
This particular composite promises application for protective products to service the electronic industry.